2. Interferon Flashcards
How does HPV affect interferon?
- Antagonists encoded by the virus stop interferon system from working well
- Interferon usually targets the virus in many ways, but now it can’t effectively
What is the IRF-7 gene and what happens if it is faulty?
- Key player in interferon induction pathway
- Inherit 2 copies of the faulty gene => doesn’t work well
- Therefore, can’t produce interferon alpha in response to infection
(heterozygous mutation can be revealed by exome sequencing)
What causes and happens to people with an autosomal recessive IFNAR2 gene condition?
- Deletion causing gene to come out of frame
- Premature stop codon inserted
- Defective interferon alpha receptor
- Can’t respond to interferon produced during infection
(even viruses that have been vaccinated against can manifest)
What is the IRF-3 gene for, and what happens if there is a heterozygous mutation?
- Part of the interferon signalling pathway
* Patients won’t respond well to viruses
What is the most common cause of sporadic encephalitis in the Western world, at what age is it most common and what causes it?
Herpes simplex encephalitis
• Most common in childhood, affecting healthy individuals on primary infection with HSV-1
• Associated with inborn errors in at least 6 genes - one or more genes in the interferon cascade are not working
• So interferon not made when the person is affected with HSV (CNS intrinsic interferon response)
• So much virus in the body => goes to part of the body that it normally wouldn’t
Are the proteins and genes involved in the interferon cascade part of our innate or adaptive immunity?
All part of our innate immunity
What are interferons?
- Signalling proteins made and released by host cells in response to viruses
- Soluble factor, which if put onto cells, induces immunity to any virus
What does interferon activate the de novo transcription of, and what does this do?
Interferon stimulated genes (ISGs) - induce anti-viral state in cells (prevents more cells getting infected)
How does giving interferon to people with a common cold make them feel?
- Even worse
* Interferon is associated with a lot of the symptoms of viral infection
What are Type I interferons and their 3 functions?
Polypeptides secreted from infected cells (include alpha and beta)
• Induce antimicrobial state in both infected and neighbouring cells
• Modulate innate response to promote antigen presentation and NK, but inhibit pro-inflammation
• Activate the adaptive immune response
What is the first interferon to be made?
IFN-beta
Describe what initially happens when cells sense a viral infection
- Cells make an interferon response - new copies of IFN-beta
- IFN-beta diffuses and interacts with receptors on neighbouring cells
- Switching on of genes in these cells to put them into an anti-viral state
What are plasmacytoid dendritic cells (PDCs)?
- Specialised cells that are good at making interferon (particularly IFN-alpha)
- Secretion of type I interferon will recruit APCs and adaptive immune cells
What proportion of cells in the body make type 1 interferons?
Nearly every cell
PDCs tend to make IFN-alpha, epithelial cells tend to make IFN-beta
On what tissues is the IFN-alpha receptor (IFNAR) present on?
All tissues
What triggers IFN-beta induction?
IRF-3
What interferon regulatory factor (IRF) do PDCs express high levels of?
IRF-7
How many genes are there for IFN-alpha and beta?
- 13-14 isotopes for IFN-alpha
* 1 gene for IFN-beta
Briefly describe interferon-gamma
- Type II IFN
- More specialist
- Produced by immune cells: activated T cells and NK cells
- Signals through IFNGR
What does IFN-λ (lambda) protect, where is it important and what IL receptors does it signal through?
- Protects the barriers of your body e.g. respiratory and gut epithelium
- Important in the liver e.g. hepatitis
- Signals through IL-28 and IL-10β receptors
Which interferons are made when epithelial cells are infected?
IFN-lambda and IFN-alpha/beta
What are polymorphism in IFN-lambda associated with?
Improved outcome from Hepatitis B (HBV) and C (HCV)
Better clearance and response to antiviral therapy
What is the one thing that a virus cannot hide, that can be detected by our cells?
Its nuclei acid
What are PAMPs?
Pathogen associated molecular patterns (unique to the pathogen)
What are PRRs?
Pattern recognition receptors -sense foreign nuclei acids via specific receptors:
• RLRs: cytoplasmic RIG-I like receptors
• TLRs: endosomal Toll-like receptors
(there are also cytoplasmic nucleotide oligomerisation domain receptors - NLRs)
Do TLRs, RIGs and cGASs detect RNA or DNA?
TLR + RIG = RNA
cGAS = DNA
Where are TLRs found in the cell?
- Membrane protein
* Found on plasma membrane and endosomal membranes
What do RLRs sense and do?
- Sense viruses in the cytoplasm
* Send signal through a mitochondrial located pathway
When a virus comes in and makes some RNA, what is this detected by and what does this do (PRR=>Mavs)?
- One of the PRRs - RIG-I or Mda-5, that sits in the cytoplasm
- Upon binding to viral RNA, they change shape and unfold to interact with a second molecule sitting on the mitochondria - Mavs (mitochondrial activated virus signaller)
What happens when Mavs are activated (Mavs => response beginning)?
- Sends a signal - phosphorylating IRF-3 and 7
- IRF-3 and 7 moves into the nucleus and attaches to the promoter of the IFN-beta gene
- New synthesis of mRNA for IFN-beta
- New IFN-beta (and alpha) made, response begins
(IFN-beta is released from these cells and travels to neighbouring cells)
What happens if a viral ssRNA is detected inside an endosome of a cell?
- TLR-7 will recognise it
- Signal to a molecule outside the endosome - MyD88
- Phosphorylate IRF-3 and 7
- Expression of IFN-alpha and beta
Are the nucleic acids of a virus exposed inside an endosome of a cell?
Yes (at some point in their life cycle they will be inside an endosome too)
What enzyme detects viral DNA in the cytoplasm, and how does it lead to the production of IFNs?
cGAS
• binds to DNA (double stranded)
• synthesises cGAMP
• cGAMP binds to STING in the endoplasmic reticulum
• STING moves into the nucleus to transcribe IFNs (like IRF-3/7)
What effect is IFN-beta acting on cells called?
Paracrine effect - on neighbouring cells
Autocrine effect - on same cell
What kind of receptor is the IFN-alpha/beta and gamma receptor?
Alpha/beta - Heterodimeric receptor composed of IFNAR1 and IFNAR2
Gamma - Heterodimeric receptor composed of IFNGR1 and IFNGR2
Describe everything that happens when an IFN binds to the IFN-alpha/beta receptor
- Activates Jak and Tyk
- This phosphorylates STAT1 and STAT2
- STAT molecules dimerise and combine with IRF9
- This goes to the nucleus and binds to a responsive promoter region
What do the following interferon stimulated genes do: • Protein kinase R • 2'5'OAS • Mx • ADAR • Serpine • Viperin
- Protein kinase R - inhibits translation (host makes no new host or viral RNA - cell and viral death)
- 2’5’OAS - activates RNAse L - destroys ssRNA
- Mx - inhibits incoming viral genomes
- ADAR - induces errors during viral replication
- Serpine - activates proteases
- Viperin - inhibits viral budding
What is IFN-induced transmembrane protein (IFITM3) and why is it important?
• ISG (interferon stimulated gene)
• Sits on the membrane of endosomes, in cells stimulated with IFN
• When IFITM3 is overexpressed, the membranes of the endosomes become very rigid
- virus cannot escape
• e.g. flu virus enters cell through endosome so IFITM3 can prevent injection of DNA into nucleus
Where is IFITM3 deficiency common and what does this mean?
- Parts of Asia (China and Japan)
* More risk of hospitalisation
How does Mx1 work?
- Sits on cytoplasm of cells
- Traps viruses just as they are releasing their nuclei acid
- Forms multimers which wrap around the nucleocapsids of incoming viruses
- Useful for flu viruses
What is Mx2 important in?
Prevents injection of HIV RNA into the nucleus of a cell
What protein family is Mx a member of?
Dynamin and large GTPases
How long does the antiviral state sustained by IFN last and why?
- Several hours, maybe a day
- Ability to respond to IFN is lost due to negative feedback
- SOCS (suppressor of cytokine signalling) genes turn off the response
How have viruses evolved to evade the IFN response?
- Hiding the PAMP (inside capsule?)
- Interfere with host cell gene expression
- Block IFN induction cascades - destruction or binding
- Block individual IFN induced antiviral enzymes
- Activate SOCS
- Develop a replication strategy that is insensitive to IFN e.g. replicate very quickly
How does HCV control/evade IFN?
Has NS3/4 protease - acts as antagonist to IFN induction by cleaving Mavs
How does influenza control/evade IFN?
Has NS1 protein - acts as antagonist to IFN induction by binding to and preventing formation of RIG-I/TRIM25/RNA complex - prevents activation of signalling pathway
Also can enter the nucleus and stop IFN-beta mRNA from being exported
How do Pox viruses interfere with IFN?
Large DNA virus
• More than half of genome is comprised of accessory genes that modify immune response
• Encode soluble cytokine receptors which mop up IFN
• IFN can’t interact with receptor
• IFN and other cytokines are produced in abundance and contribute to the pathology
How does the Ebola virus interfere with IFN?
• RIG-I and Mda-5 sense the virus
• Ebola produces:
- VP35 - blocks signalling cascade, inhibits the RIG-I pathways
- VP24 - blocks the signal from the IFN receptor into the nucleus, STAT1 can’t enter nucleus (ISGs can’t be transcribed)
How does the effect of a virus that has co-evolved with a host for a long time, compare with a virus that has recently crossed from an animal species?
- Co-evolved - balance between its own genes and what the host is trying to do to the virus - controlled and not a lot of bystander damage by immune response
- New virus - lots of responses switched on, damaging the host cells - nothing in harmony
Can interferon be dangerous?
- Viruses can skew the immune response to increase their own replication and transmission
- Too much interferon can be released - triggers pro-inflammatory pathways
- High levels of IFN is accompanied by massive release of TNF alpha and other cytokines e.g. in dengue haemorrhagic fever
- Virus is not controlled either, which leads to more IFN release
- Levels can vary from protective to immunopathologic
- Patient can succumb to immune pathology rather than virus
How can a cytokine storm affect the lungs?
Accumulation of immune cells in the lung spaces => pulmonary fibrosis
Why isn’t it common to use interferon as a broad spectrum antiviral?
- Bad if there is too much (dose has to be right)
* Many unpleasant side effects - stimulated TNF-alpha, IL-6 etc. which makes the patient feel awful
Why could it be useful to use IFN-lambda as an influenza therapeutic agent?
- Only present on epithelial surfaces
- Cannot signal through receptors present on immune cells - NO IMMUNE RESPONSE
- Don’t have side-effects of immunopathology and inflammation
- Can still induce antiviral state in epithelial target cells
When can cells deficient in an IFN response be useful in medicine?
Used to grow attenuated virus strains for vaccines
What is the downside to engineering a virus for a vaccine, in a way that it can’t control the IFN system, and what is the solution to this?
- Difficult to propagate the virus sufficiently to produce enough virus for loads of vaccines
- Solution: culture cells that are genetically engineered to be deficient in the IFN response
How can we take advantage of the fact the cancer cells are in an IFN deficient state?
Oncolytic viruses
• Give them a virus throughout the body
• Virus won’t be able to fight the IFN response
• Only replicates in and kills cancer cells, that are IFN deficient
